Electrohydraulic drive units, which—constructed as linear drives—respectively comprise at least one cylinder-piston arrangement that can be pressurized in controlled manner by a hydraulic pump and in particular are suitable as machine drives are known in various configurations. In this regard, reference may be made, for example, to DE 102014005352 A1, DE 102012013098 A1, DE 102009052531 A1, DE 4036564 A1, DE 102005029822 A1, DE 4314801 A1, WO 2012/112130 A1, WO 2011/003506 A1, EP 103727 A1, DE 102014218887 B3 and DE 202015106161 U1.
An electrohydraulic drive unit of the said generic type can be inferred in particular from the last cited DE 202015106161 U1. One of the characteristics then consists in the fact that the hydraulic pump together with its working port may be connected optionally to each of the two hydraulic working chambers of the—double—acting—cylinder-piston arrangement. Hereby the piston of the cylinder-piston arrangement may be moved—by appropriate pressurization of one of the two hydraulic working chambers from the hydraulic pump—actively in each of the two directions of movement (lowered as well as raised in the case of vertical axis of movement). In a typical application of such an electrohydraulic drive unit, a first part of the downward movement of the piston (the so-called rapid traverse) during a working cycle, with the servo-suction valve open, takes place due to gravity alone with displacement of hydraulic fluid from the second hydraulic working chamber into the tank, wherein the displacement is braked by the hydraulic pump switched to braking mode. Following a changeover phase, which typically runs shortly before the tool is set on the workpiece during use of the drive unit in a press, the second part of the downward movement of the piston (the so-called power mode) takes place, as does the subsequent holding of the piston at the bottom dead center under pressurization of the first hydraulic working chamber from the hydraulic pump in its pumping mode, wherein hydraulic fluid is displaced from the second hydraulic working chamber against an opposing pressure generated by a pressure-holding valve into the tank during the power mode.
In various applications, the piston of the cylinder-piston arrangement is under considerable tension at its bottom dead center. This is the case, for example, during application of the respective electrohydraulic drive unit in a straightening or bending press or in a press brake, in which the workpiece to be formed—depending on its material properties and dimensions—typically exerts on the piston, at the bottom dead center thereof, a high opposing force directed against the piston movement causing the forming process. Accordingly, in such applications, the first hydraulic working chamber of the cylinder-piston arrangement is under considerable pressure at the bottom dead center of the piston. In order to dissipate this pressure before the piston—due to pressurization of the second hydraulic working chamber—is actively raised, a so-called decompression phase following the holding phase is provided according to DE 202015106161 U1. For this purpose—without change of the connection of the first hydraulic working chamber of the cylinder-piston arrangement to the working outlet of the hydraulic pump—the directions of rotation and flow of the hydraulic pump, which in the forming and holding phase pressurizes the first hydraulic working chamber, are reversed. The return flow of hydraulic fluid from the first hydraulic working chamber via the hydraulic pump—now operating in braking mode—to the tank is then limited, according to DE 202015106161 U1, via a flow throttle. The latest end of the decompression phase is then dictated by the process itself, namely at the latest at the point of equilibrium between the forces acting on the piston (especially hydraulic forces, weight forces, reaction forces or resilience forces of the workpiece, resilience forces of the machine parts deformed elastically during pressing), wherein the tool is still typically bearing on the workpiece at this time. After decompression in this sense has taken place, reversal of the hydraulics is commanded in the sense of pressurization—causing active raising of the piston—of the second hydraulic working chamber by the hydraulic pump, which has been switched back to pumping mode.
The present invention has taken on the object of providing an electrohydraulic drive unit of the generic type that is characterized by further improved operating behavior, especially in the region of reversal of movement of the piston of the hydraulic cylinder-piston arrangement.